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The aim of the present work was to further study the characteristics of stimulation of growth of wild type and mutant cells by various auxins. The ability of the mutant to form auxinconjugates was investigated as a possible basis for increased auxin tolerance. An auxin analog specifically triggering cell death was identified by differential screening on wild-type and mutant cells. MATERIALS AND METHODSAn auxin requirement for the stimulation of cell proliferation has been demonstrated for many types of tissues grown in vitro. Recent work on the transformation of tissues by Agrobacterium tumefaciens have shown that the oncogenous transferred bacterial DNA encodes for two genes involved in a pathway of auxin biosynthesis, thus abolishing the requirement of exogenously supplied auxin for the growth of transformed cells (13,22). The mechanism by which auxin triggers cell proliferation and differentiation is still far from being understood. Specific auxin receptors have been postulated but no proof for their function in mediating auxin actions has been given (1 1).Cells derived from mesophyll protoplasts of tobacco and grown at low densities were found to be killed by relatively low auxin concentrations (5 FAM NAA2 or IAA) (6). A correlation between such toxicity and auxin conjugation was also observed in these 'Supported in part by a grant from the Ministere de l'Industrie et de la Recherche, "GEn&tique et physiologie des veg6taux sup6rieurs." 2Abbreviations: NAA, naphthaleneacetic acid; DAST, diethylaminosulfur trifluoride; P-cell, protoplast-derived cell; PE, plating efficiency; picloram, 3,5,6-trichloropicolinic acid; RPE, relative plating efficiency.

Auxin induction of the proliferation of Nicotiana tabacum (cv Xanthi) mesophyll protoplasts and of protoplast-derived cells was studied. The growth-promoting properties and cytotoxicities at high concentrations of IAA and naphthaleneacetic acid were strongly affected by cell density. The induction of growth by 2,4-dichlorophenoxyacetic acid and picloram was not affected by cell density. The comparison of catabolism of these "'Cllabeled auxins by protoplasts showed that IAA and naphthaleneacetic acid were rapidly accumulated and conjupted unlike 2,4-dichlorophenoxyacetic acid and picloram. The major catabolite derived from naphthaleneacetic acid was identified as naphthaleneacetyl-L-aspartate. The biosynthesis of this conjugate in protoplasts was inducible by naphthaleneacetic acid concentrations found to be cytotoxic under low density growth conditions. However, although it was taken up by cells, the conjugate was not cytotoxic at concentrations as high as 0.2 mM under low density growth conditions. The relationship between conjugation processes and auxin cytotoxicity is discussed.

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